U.S. Pat. No. 4,112,515 discloses a technique for mixing a carrier gas and a catalyst fluid for curing foundry molds and cores in which the catalyst fluid is vaporized before it is mixed with the carrier gas. The desired proportion between the catalyst gas and the carrier gas is arrived at in this prior patent by sensing the flow of the carrier gas and adjusting a variable valve in the flow path of the catalyst gas to achieve the proper proportion therebetween. The technique of U.S. Pat. No. 4,112,515 is regarded as an improvement over the technique of U.S. Pat. No. 3,919,162 in which the catalyst is still in its liquid phase when it is mixed with the carrier gas. However, the above-described technique of U.S. Pat. No. 4,112,515 may not respond quickly or accurately enough to load conditions to consistently regulate the proper porportion of the gases in the mixture. Load changes are sometimes not sensed quickly enough by the comparator mechanism, thus producing a time lag in the proportioning apparatus. The comparator of the prior art system relies upon sensing gas flow. Such sensing can be delayed because of the intertia and momentum characteristics of the gas flow, thus delaying response of the comparator to changing load conditions.